1. Field of the Invention
The invention relates to the process of fabricating semiconductor chips. More specifically, the invention relates to a method and an apparatus for using results of a simulation to select evaluation points for use in a model-based optical proximity correction (OPC) operation.
2. Related Art
As integration densities on a semiconductor chips continue to increase at an exponential rate, it is becoming progressively harder to deal with optical effects that arise during the optical lithography process involved in manufacturing the semiconductor chips. These optical effects can cause unwanted distortions in the printed layout that is generated by the optical lithography process.
To remedy this problem, a layout is often subjected to a model-based optical proximity correction (OPC) operation, which adjusts the layout to compensate for optical effects. (Although the term “optical proximity correction” is used, more generally the term as used herein refers to correction for any specified proximity effects, e.g. optical, micro-loading, etch, resist, etc.) These adjustments are made based upon results of model-based simulations of the printed layout. During this OPC operation, edges in the layout are divided into segments, and each segment is adjusted with a negative or a positive bias based upon a deviation between the desired layout and the simulated layout at an “evaluation point,” which is located within the segment.
The location of this evaluation point can be pre-determined through simple rules, such as by taking the mid-point of the segment, or by considering other factors, such as the shape of the feature to which the segment belongs. Unfortunately, these simple rules frequently do not select the optimal evaluation point for a given segment.
Even though the simulated layout may match the desired layout at a given evaluation point, environmental factors may cause the simulated layout to deviate significantly from the desired layout at other points on the segment. Note that this deviation will not be detected by taking measurements at the given evaluation point. Hence, the deviation will not be corrected during the model-based OPC process.
Hence, what is needed is a method and an apparatus that facilitates selecting optimal evaluation points for a model-based OPC process.